346 Pt'of. Owen's Description of the Lepidosiren annectens. 



and third branchial arches; and the corresponding- arterial trunks* undergo 

 no subdivision as they wind round them, but are continued entire, as in the 

 Amphiuma and Menopoma, to their termination at the opposite side of the vas- 

 cular circle. The branches which afterwards unite to form the single pulmo- 

 nary artery on each side are given off from near the termination of the se- 

 cond and third pairs of the primitive aortic trunk ; which thus combine the 

 functions of both systemic and pulmonary arteries. 



The branchiae of the Lepidosiren resemble in form those of the Siren, con- 

 sisting of separate elongated filaments, attached only by one extremity to the 

 branchial arch ; but these extremities are fixed directly to the branchial arch, 

 and not to a common pedicle extended therefrom, as in the Siren. Viewed 

 with a moderate lens the tripinnatifid structure is beautifully seen in each 

 branchial filament. The first gillf consists of a single row of fourteen of these 

 subcompressed filaments, each of which is about one line in length and a 

 third of a line in breadth. The second gillj, which is developed, as before 

 stated, on the fourth branchial arch, is the largest, and consists of a double 

 row of fifteen branchial filaments. The third gill§ has a similar structure. 

 The fourth gill || consists, like the first, of a single row of fourteen tripinnatifid 

 filaments, which are shorter and smaller than those of the first. 



The cartilaginous branchial arches are developed on each side in the sub- 

 mucous tissue, and, as before stated, are not attached either to the hyoid ap- 

 paratus below, or to the cranium above. The membrane covering the 3rd, 

 4th, and 5th arches is minutely papillose. The first branchial aperture or 

 interspace^ is a narrow slit three lines long, and is defended by a series of 

 minute denticulations projecting from the branchial arch. Bristles are repre- 

 sented as passing through the five branchial interspaces in Tab. XXV. fig. 3. 

 The second aperture** is the widest; it is five lines long, and its margins are 

 smooth : the third aperture is also five lines long, but is narrower than the 

 second : small cartilaginous teeth are developed from each of its margins, as 

 in the first gill-aperture : the fourth and fifth apertures present the same 

 structure but diminish in size. 



Thus the branchial current, which flows through the interspaces of those 



* Tab. XXVI. fig. 2, 2 & a. t lb. fig. 2, 1. J lb. fig. 2. 4. § lb. fig. 2, 5. 



II lb. fig. 2, 6. ^ lb. fig. 1, 1. ** lb. fig. 1, 2. 



